A film used in such projection comprises a main track which occupies the major fraction of the width of the film which has the images to be projected following one another thereon. Suitable means are provided for causing the main track to run image-by-image between a source of intense light and an objective lens. From the recorded image as illuminated in this way, the lens projects an image on a screen. To the side of said main image-carrying track, the film includes at least one auxiliary recording track, which in practice is a soundtrack conveying a sound signal, but which could equally well convey some other type of auxiliary signal, for example a clock signal or a synchronizing signal, etc. . . .
Recording on said track is performed optically, i.e. the recorded signal is represented by an optical density. It is present practice for a soundtrack to comprise a transparent central fraction of its width together with two opaque fractions extending symmetrically on either side thereof. The signal is constituted by the variable width of the transparent central fraction.
In order to read such a track, it is caused to run at constant speed between a light-emitting head and a photoelectric cell which provides an electrical signal representative of the sound signal to be reproduced. Such a signal may be amplified and applied to loudspeakers in a cinema.
The light-emitting head illuminates the film by forming a read window whose length runs across the width of the auxiliary track and whose width extends in the direction of film displacement.
The auxiliary signal is written, i.e. recorded, in an analogous manner using a light-emitting head in conjunction with a read window which is illuminated over a fraction only of its length, with said fraction being modulated by said signal.
The passband of such a device, i.e. the band of frequencies which it is capable of reproducing, falls off with increasing width of the read window. It is therefore desirable to reduce the width of said window. This effect is due to the fact that the speed at which the film runs is determined by other constraints, and to the fact that the sensitive area of the photo-electric cell extends over a greater width than the width of the above-defined read window, thus ensuring that the width of the photo cell is not a limiting constraint.
One known technique for reducing the width of the read window is to use a light-emitting head including a relatively powerful light source which illuminates a diaphragm having a slit therein. An image of the slit illuminated in this way is then formed on the film by an objective lens. In this prior apparatus, the slit constitutes the above-defined read window.
This prior device suffers from two drawbacks in particular. Firstly, the optical quality of the objective lens makes it impossible to obtain a window which is as narrow as could be desired, even if the slit through the diaphragm is itself sufficiently narrow. This limits the passband of the device. Further, given the limited sensitivity of the photo cell, the light source must be very powerful indeed since only a small fraction of its light passes through the narrow slit through the diaphragm. This gives rise to energy consumption and heating which may be objectionable.
Specific objects of the present invention include:
increasing the passband of a read and/or write device for use with an auxiliary track of a motion picture film (and in particular with a soundtrack); and/or
reducing the electrical power consumed and/or the heat power given off by such a device; and/or
making adjustment of such a device easy; and/or
making said adjustment independent of the wavelength of the light used; and/or
facilitating the insertion of such a device in an existing motion picture apparatus by installing in said apparatus a light-emitting head which is compact and/or easy to manufacture.